1. Field of the Invention
The present invention relates to an image scanner. More particularly, the present invention relates to a hand-held image scanner which incorporates a rotary encoder.
2. Description of the Related Art
An image scanner is widely used for reading an image of a document. For convenience of handling, an image scanner is proposed which is held with one hand of the user for image reading.
FIG. 6 illustrates a prior art hand-held image scanner 1. For operation, the scanner 1 may be connected to a personal computer for example. In use, as the user moves the scanner 1 over an original document P in the direction indicated by an arrow A, a line sensor incorporated in the scanner 1 reads an image (or images) carried on the document P. As shown in the figure, the scanner 1 has an elongate box-shaped body 2 which may be made of resin for example. The body 2 has one end provided with a cable 3 for connection to a non-illustrated personal computer. Further, the body 2 has an upper surface 2a provided with a switch 4 adjacent to the other end of the body 2.
FIG. 7 is a bottom view, partially cut away, of the scanner 1, whereas FIG. 8 is a transverse section of the scanner 1. As shown in these figures, the body 2 accommodates a substrate 12 on which a line sensor 26 is mounted. The line sensor 26 comprises a plurality of light receiving elements (photo-electric elements) arranged in a row extending longitudinally of the body 2. The row of light receiving elements has a length corresponding to the image reading width (which is at least equal to but normally greater than the width of the original document P). The body 2 further accommodates a light source 25 (e.g. an LED array) for illuminating the document P. A transparent glass plate 14 is provided at the bottom surface 2b of the body 2. Between the transparent plate 14 and the line sensor 26 is provided a lens array 13 for collecting light reflected from the document P to form an actual-size erect image at the line sensor 26. The body 2 further includes another substrate (not shown) on which an IC chip (not shown) is mounted as a controller for processing the image data from the line sensor 26.
Further, the body 2 is provided with a shaft 6 extending longitudinally of the body 2 and supported therein for rotation. A plurality of rollers 7 are attached on the shaft 6 for rotation therewith. Each of the rollers 7 partially projects outward from the body 2 through an opening 8 formed at the bottom surface 2b of the body 2. As a result, when the scanner 1 is placed on the document P, the rollers 7 come into contact with the document P.
As shown in FIG. 7, the shaft 6 is connected to a rotary encoder 11 via a first and a second gears 9 and 10. Specifically, the first gear 9 is attached to one end 6a of the shaft 6, whereas the second gear 10 is connected to the rotary encoder 11 while engaging the first gear 9. With this structure, the rotary encoder 11 detects the rotational angle or the number of revolutions of the rollers 7.
As shown in FIG. 9, the rotary encoder 28 includes a rotary disc 16 of a predetermined diameter mounted on the same shaft as the second gear 10 is mounted (see FIG. 7). The rotary encode 28 also includes a light emitting element 17 and a light receiving element 18, both housed in a non-illustrated case in facing relationship to each other with the rotary disc 16 interposed therebetween. The rotary disc 16 is formed with a plurality of radially extending slits 19 having a predetermined width. With this structure, as the rotary disc 16 rotates, light emitted from the light emitting element 17 passes through each slit or is blocked by the portion between adjacent slits. The light receiving element 18 detects the light passing through the slits and transmits the detection signals to the controller. Based on the detection signals, the controller determines the rotational angle or the number of revolutions of the rollers 7.
Thus, the above-described scanner 1 is capable of scanning the document image line by line by detecting the number of revolutions of the rollers 7 indicative of the scanning distance of the body 2.
Specifically, while the rollers 7 rotate in contact with the document P in scanning the document image, the rotary encoder 11 detects the rotational angle or the number of revolutions of the rollers 17. Based on the detection signals outputted from the rotary encoder 11, the controller figures out the scanning distance of the body 2 and outputs a timing signal to the line sensor 26 every time the body 2 advances a distance corresponding to one line. The line sensor 26 reads the document image for each line and outputs the image data to the controller upon receiving the timing signal from the controller.
For example, when the resolution is 300 dpi, one line which corresponds to one dot has a width of 0.085 mm (calculated according to the equation “Width=25.4 mm÷300”). Therefore, the controller outputs a timing signal to the line sensor 26 every time the body 2 advances 0.085 mm. The line sensor 26 successively outputs image signals for each line to the controller every time it receives a timing signal.
The controller successively transmits the image signals for each line received from the line sensor 26 to the non-illustrated personal computer for example. The image data is stored in the memory of the personal computer for reproducing a two-dimensional image.
With the above-described structure, it is necessary to enhance the detecting resolution of the rotary encoder 11 for increasing the resolution of the scanner 1. For this purpose, the number of slits maybe increased by narrowing the intervals between the slits 19 or by using a diametrically larger rotary disc.
However, when the interval between the slits is narrowed, the light receiving element may erroneously detect light passing through a wrong slit adjacent to the proper slit. To avoid such erroneous light detection, it is necessary to use a high-precision light emitting/receiving elements, which are however generally expensive.
On the other hand, the use of a diametrically larger rotary disc results in a corresponding increase in the size of the scanner 1, as shown in FIGS. 10 and 11. In these figures, the housing of the scanner is partially omitted for showing the structure within the body 2. As is clear from these figures, the rotary disc 16 is mounted on a support member 24 provided at one end 2c of the body 2 for rotation about a shaft 16a. The scanner 1 also includes a photointerrupter 20 which has a pair of light emitting and light receiving elements (not shown) facing each other with the rotary disc 16 interposed therebetween. Further, the shaft 16a of the rotary disc 16 carries a first pulley 21, whereas a second pulley 22 is attached on a non-illustrated shaft of the rollers. The first and second pulleys 21, 22 are connected to each other via a belt 23 wound therearound, thereby connecting the shaft of the rollers to the rotary disc 16.
As appreciated in FIGS. 10 and 11, the rotary disc 16 projects largely in the thickness direction of the substrate 12. As a result, the size of the scanner 1 increases correspondingly, which is inconvenient for use.